This invention relates to an inlet port for an internal combustion engine which has an inlet hole opened on the end face of a cylinder chamber at an off-center position, the inlet hole being provided with an inlet valve and connected to an inlet passage.
Conventionally, the following types of inlet port were usually employed: a helical inlet port in which the connecting end of an inlet passage to an inlet hole forms a spiral section which gets closer to the inlet hole while turning round the valve stem of the inlet valve, and a tangential inlet port in which the connecting end of an inlet passage to an inlet hole is positioned along the tangential line from a point on the wall surface of the cylinder chamber closest to the inlet hole.
When these types of inlet port are compared, the helical inlet port can generate strong swirl inside the cylinder chamber because it has the spiral section at the connecting end of the inlet passage to the inlet hole but the pressure loss of the inlet passage is large and therefore the volume efficiency of the cylinder chamber is low. On the other hand, the pressure loss of the inlet passage is small and the volume efficiency of the cylinder chamber is high for the tangential inlet port because it has no spiral section in the inlet passage but the swirl which is generated in the cylinder chamber can not be made strong. Then, in the tangential inlet port, since the valve stem of the inlet valve is inserted through the center of the connecting end of the inlet passage to the inlet hole and the stem guide of the inlet valve is extended, the inlet air flow from the inlet passage to the cylinder chamber collides with the valve stem to be divided into two streams. Then, the divided streams, after passing the position of the valve stem, collide and interfere with each other. As a result, the swirl inside the cylinder chamber is further decreased. Besides, the dividing ratio of the above inlet air flow is not constant and the swirl becomes unstable.
Therefore, in the conventional types of inlet port, it is not possible to generate a suitable level of stable swirl without reducing the volume efficiency.